A spot size converter (sometimes referred to as a mode size converter or mode size adapter) is an optical device which is capable of expanding or contracting a mode of light in one or more spatial dimensions. For example, a spot size converter may expand a mode of light passing through a waveguide included in a planar lightwave circuit (PLC) (sometimes referred to as a photonic integrated circuit (PIC)) to a size which fits a mode of an optical fiber. Similarly, the spot size converter may contract a mode of light passing through the optical fiber to a size which fits the mode of the waveguide included in the PLC. Here, spot size conversion is needed in order to efficiently couple the PLC and the optical fiber such that an amount of loss caused by the coupling is reduced.